1. Field of the Invention
This invention relates to a linear motion rolling guide unit applied to linearly sliding portions of a machine tool, and various types of assembling apparatuses and test apparatuses, and comprises a track rail provided with raceway surfaces, and a slider adapted to be slid relatively on the track rail via rolling elements.
2. Description of the Prior Art
A linear motion rolling guide unit has heretofore demanded a high accuracy with the advance of the mechatronic techniques, and, moreover, the application field of this guide unit has expanded with the development of the techniques. A linear motion rolling guide unit is being used widely for various kinds of mechanical apparatuses. A linear motion rolling guide unit must meet the speed increasing, miniaturizing and load capacity increasing requirements, have a long life, generate little vibration and noise during the travel of a slider, and position the slider with a high accuracy.
A known linear motion rolling guide unit is shown in FIG. 6. In this linear motion rolling guide unit, a slider 1 is saddled on a track rail 2 and slid freely thereon via rolling elements 7 circulatingly moved along raceway surfaces 4 of the track rail 2. The track rail 2 is provided with the raceway surfaces 4 on both of longitudinal side surfaces 3 thereof. The track rail 2 is also provided in a longitudinally extending upper surface 14 thereof with mounting holes 13 which are spaced from each other. The track rail 2 is fixed on a mounting base 20, such as a bed, a machine base and a working base. Bolts are inserted into the mounting holes 13 formed in the track rail 2, and the screwed to threaded holes formed in the mounting base 20, whereby the track rail 2 is fixed to the mounting base 20. The slider 1 has a casing 5 movable relatively to the track rail 2, and end caps 6 fixed to both ends of the casing 5. The casing 5 is provided with mounting holes 19 used to mount a machine and a tool, a mechanical part, a chuck and a gripping unit on the casing. The casing 5 and end caps 6 are provided in their lower surfaces with recesses 10 at which the casing 5 and end caps 6 are saddled on and moved along the track rail 2, and raceway surfaces 9 are formed on the portions of the opposed surfaces of the recesses 10 which correspond to the raceway surfaces 4 of the track rail 2. Rolling elements 7 comprising balls are inserted in raceways formed by the opposed raceway surfaces 4, 9 so that the rolling elements roll therein. In order to prevent the rolling elements 7 from falling from the casing 5, retainer bands 18 are fixed to the casing 5 so as to surround the rolling elements 7. In order to seal clearances between the track rail 2 and slider 1, lower seals 8 are provided on lower surfaces of the slider 1.
The end caps 6 are provided at both sides thereof with claws by which the rolling elements 7 are scooped from the raceway surfaces 4 forming loaded raceways with respect to the track rail 2, and direction changing passages for changing the direction of advance of the rolling elements 7 and circulating the rolling elements properly. Side seals 17 which attain the sealing of clearances between the track rail 2 and both of longitudinal end portions of the slider 1 are provided on the end caps 6. The end caps 6 are fixed to both end surfaces of casing 5 by screws 25 inserted into fixing holes. The rolling elements 7 in loaded regions and rolling on the raceway surfaces 4 of the track rail 2 are guided into the direction changing passages formed in the end caps 6, and then moved to return passages 12 formed in upper portions of the casing 5 so as to extend in parallel with the raceway surfaces 9, the rolling elements 7 thus rolling in endless circulating passages. Owing to the rolling of the loaded rolling elements 7 in the raceways formed between the raceway surfaces 9 formed on the slider 1 and those 4 formed on the track rail 2, the track rail 2 and slider 1 can be moved smoothly and relatively to each other. In the linear motion rolling guide unit, grease or a lubricating oil is used as a lubricant. When the lubricant comprises grease, it is supplied from grease nipples 11 to the passages in which the rolling elements roll to carry out the lubrication of the rolling elements, and, when the lubricant comprises a lubricating oil, pipe joints are provided instead of the grease nipples 11, through which the lubrication of the rolling elements is carried out.
In a conventional linear motion rolling guide unit, the rolling elements in a slider are circulated endlessly when the slider is moved slidingly on a track rail. When the rolling elements are in loaded raceways between the track rail and slider and roll as they support a load, a contact of adjacent rolling elements occurs. This phenomenon causes the roundness of the rolling elements and the flatness of the track rail to be lost, and an external load to be imparted partially to the rolling elements, so that the rolling elements are deformed. Consequently, a distance between the centers of adjacent rolling elements varies delicately, and the rolling elements come closer to each other, and contact each other forcibly at times. Since the rolling direction of the rolling elements is the same, the rolling directions of the portions of adjacent rolling elements which forcibly contact each other become opposite to each other, and a force occurring due to the contact of the rolling elements works so as to prevent a smooth rolling of the rolling elements. When the slider is moved slidingly in this condition, the rolling elements slide and roll repeatedly, or collide with the adjacent rolling elements and raceway surfaces repeatedly, and noise and vibration of a comparatively high frequency, or early wear in the rolling elements occur. This causes a decrease in the guiding accuracy and lifetime of the linear motion rolling guide unit.
There are known linear motion rolling guide units developed so as to effect a smooth circulation of rolling elements, which are disclosed, for example, in Japanese Patent Laid-Open Nos. 242126/1987 and 173946/1994.
In the linear motion rolling guide unit disclosed in Japanese Patent Laid-Open No. 242126/1987, the rolling elements are arranged in their respective positioning holes so that the rolling elements do not contact one another, and these rolling elements are held by retainers having projecting portions so that the rolling elements do not fall from the positioning holes. In the linear motion rolling guide unit disclosed in Japanese Patent Laid-Open No. 173946/1994, spacers of a synthetic resin the sizes of which are smaller than the diameter of rolling elements are provided between adjacent rolling elements which roll in circulating passages.
These linear motion rolling guide units can prevent adjacent rolling elements from contacting each other but the rolling elements are arranged in an equally spaced manner. When the rolling elements are arranged in an equally spaced manner with the slider received a high load, such as an impact, impressions, i.e. elastic deformation occurs at this moment in the positions in the raceway surfaces of the slider and track rail which the rolling elements contact since the rolling elements have generally the highest hardness among the constituent parts of the linear motion rolling guide unit. The impressions necessarily occur at the intervals at which the rolling elements are arranged. Accordingly, even when the rolling elements change their places successively in accordance with the sliding movement of the slider with respect to the track rail, groups of rolling elements arranged at regular intervals fit simultaneously in the impressions formed at the same intervals, and such actions are made repeatedly. This causes vibrations to occur during the sliding movement of the slider, and the rolling elements to be early worn.
When a plurality of rolling element circulating passages are formed in a linear motion rolling guide unit in which rolling elements are arranged at equal intervals, the rolling elements rolling in the circulating passages enter the loaded regions of the passages simultaneously. Therefore, every time the rolling elements enter the loaded regions of the circulating passages the slider slidingly moved on the track rail is caught, so that the traveling accuracy of the slider is adversely affected.
Even in the structure of a conventional linear motion rolling guide unit described above which is provided with spacers among the rolling elements with impressions formed in the raceway surfaces due to the equally spaced rolling elements or with a plurality of rolling element circulating passages provided, the regular rolling of the rolling elements causes noise and vibrations to occur, or the rolling elements to be early worn, or the traveling accuracy of the slider to be deteriorated.